With development and popularization of electronic devices, various charge methods have been applied, including conventional charge methods using specific adapters and newly proposed charge methods using interfaces such as USB. For mobile electronic devices, they are typically charged by connection to PCs or notebook computers. For example, a mobile phone can be charged by directly coupling it to a PC or notebook computer. For some smart phones, however, in order to charge them, computers must be powered on and cannot be in a sleep state. This is because such smart phone needs to determine whether to initiate a charging process based on status of two differential signal lines of a downlink USB interface of the computer. A voltage exists on the two differential signal lines only when the computer is powered on and not in a sleep state. The smart phone will start a charging operation only if it detects a connection mode. When the computer is powered off or in a sleep state, no status signal is outputted from the two differential signal lines of the downlink USB interface. As a result, the smart phone will not detect a connection mode, and thus will not start a charging operation.